Apoptosis is a normal physiologic process that leads to individual cell death. This process of programmed cell death is involved in a variety of normal and pathogenic biological events and can be induced by a number of unrelated stimuli. Changes in the biological regulation of apoptosis also occur during aging and are responsible for many of the conditions and diseases related to aging. Recent studies of apoptosis have implied that a common metabolic pathway leading to cell death may be initiated by a wide variety of signals, including hormones, serum growth factor deprivation, chemotherapeutic agents, and ionizing radiation. Wyllie (1980) Nature, 284:555-556; Kanter et al. (1984) Biochem. Biophys. Res. Commun., 118:392-3999; Duke and Cohen (1986) Lymphokine Res., 5:289-299; Tomei et al. (1988) Biochem. Biophys. Res. Commun., 155:324-331; and Kruman et al. (1991) J. Cell. Physiol., 148:267-273.
Although agents that affect apoptosis have therapeutic utility in a wide variety of conditions, it has not been possible to screen for these agents based on their apoptotic modulating activity. Such assays require a cell strain that can be maintained in vitro and retain sensitivity to apoptosis modulating signals. The vast majority of cell lines used to screen agents are selected for their ability to be maintained in vitro. Cells that are most easily maintained are "transformed" cells that have lost the ability to undergo apoptosis and are thus unsuitable for use in screening apoptosis modulating agents. Although cell strains such as the mouse embryonic C3H-10T1/2 type have been shown to be sensitive to such agents, it has not been possible to perpetuate a phenotypically stable strain of these cells beyond 120-140 population doublings. Thus, it has been impossible to obtain the reproducible results necessary to engineer a high through-put replicate culture assay for drug screening.
It has now been found that, by the method described herein, cell strains sensitive to apoptotic agents can be used to provide reproducible results in screening for agents that modulate a wide variety of disorders. These phenotypically stable cell strains can now be maintained for at least 900 population doublings. Moreover, by utilizing the parameters of the screening assay, new, improved cell strains can now be obtained that are suitable for use in the screening assays and for studying apoptosis.